Cracking furnaces are used in particular in the production of ethylene. In the steam cracking process for ethylene, a hydrocarbon feedstock is diluted with steam, and then heated rapidly to a high temperature by passing it through tubes (usually referred to as “furnace coils”) in a furnace. The high temperature decomposes the hydrocarbon feedstock. The output stream, containing a broad mixture of hydrocarbons from the pyrolysis reactions in the pyrolysis tubes plus unreacted components of feedstock, is then quenched to prevent recombination of the products. The cooled stream can then be processed through a series of distillation and other separation operations in which the various products of the cracking operation are separated.
Known cracking furnaces suffer from a number of problems. Because of the very low residence time of the feedstock and steam flowing through the tubes in the furnace (a few tenths of a second), the furnace and the tubes must be maintained at a very high temperature in order to achieve the necessary rapid heating to achieve pyrolysis. A large amount of fuel is thus required to fire the furnace.
Further, the very high temperature of the tubes in the furnace leads to the deposition of coke on the inside of the tubes. This coking is particularly unwelcome, as the presence of a layer of coke on the inside of the tube reduces heat transfer from the furnace to the feedstock, and so affects yield. It also increases the pressure drop in the pyrolysis tube, which also reduces yield.
If the coke deposition is sufficiently severe, it is normally necessary to take a furnace out of service periodically (typically every 20 to 60 days) to allow decoking of the tubes (such as by steam cleaning). Since each furnace represents a very large capital investment, it is desirable to keep such downtime to a minimum.
In U.S. Pat. No. 6,481,492 there is a proposed design of pyrolysis tube consisting of a tube of circular cross-section divided into two flow passages by a twisted baffle. The intention is to promote lateral movement of process gases in the tube, reducing the thickness of the boundary layer at the wall of each passage, so promoting the efficiency of the heat transfer between the furnace outside the tube and the gases in the flow passages. The aim is to lower the temperature of the inner surfaces of the tube and reduce coking.
However, the use of a twisted baffle in the proposed manner results in the overall cross-section or the tube being divided into two passages each with a semi-circular cross-section. This baffle creates a resistance to the process gases, reducing their flow speed through the furnace and increasing the pressure drop. The use of semi-circular flow passages is thus not optimal. The problem of increased flow resistance is acknowledged in the document, for example at column 7 lines 30-32. It is proposed to deal with the problem by providing a twisted baffle at only certain-places in the furnace tube. However, in such arrangements gas which is initially flowing along a single passage of circular cross-section encounters the front end of a baffle, which itself provides an obstruction to the flow and a potential opportunity for fouling or coke deposition. Moreover, because of the high Reynolds numbers which are involved in pyrolysis furnace flows, normal flow will reassert itself within a few tube diameters of the downstream end of a twisted baffle, so requiring several sections of baffled tube for the overall passage of a tube through the furnace, each one creating an obstruction to flow at the upstream end and a resistance to flow.
A much older proposal for the design of a pyrolysis tube in a cracker furnace was made in AU 77718/75. In this proposal it was considered that the yield of ethylene from the cracking processes could be increased by providing a pyrolysis tube which extends through the furnace in a Convoluted manner. Helical, double helical, spiral, zig-zag and wavy convolutions were proposed. It was suggested that it is desirable to maximise the surface area to volume ratio of the tube so as to facilitate the necessary heat transfer to the process gas, and that the maximum ratio is achieved by minimising the tube diameter within practical limitations. This pointed to using a smaller diameter tube with a longer streamwise length, i.e more convoluted, which would then have a larger tube surface area to tube volume ratio than a wider diameter tube which is straight. In keeping with this aim, the embodiment of AU 77718/75 shown in the drawing has a pyrolysis tube with a large helical amplitude compared to the diameter of the tube (at least twice the internal diameter of the tube insofar as this can be determined from the drawing) and a high helix angle (about 70°-80°). However, the longer length and smaller tube diameter required by such a design would lead to a higher pressure drop and be detrimental to yield.
From WO 2006/032877 and WO 2007/104952 it is proposed to provide a cracking furnace with at least one pyrolysis tube passing through the furnace, the pyrolysis tube defining a flow passage with a cross-section which is substantially circular, wherein the pyrolysis tube is formed such that it has at least one portion with a helical centreline. Similar proposals are made in Paper 191g entitled “A novel approach to ethylene furnace coil design”, by William Tallis, ColM Caro and Chinh Dang, as prepared for presentation at the 2006 AIChE Spring National Meeting in Orlando Fla. on 23-27 Apr. 2006.